Set Default Background Wallpaper for Windows 10 Desktop Using Group Policy
Use the Windows key + R keyboard shortcut to open the Run command. Type gpedit.msc and press Enter.
Select User Configuration -> Administrative Templates -> Desktop -> Desktop. Double-click the Desktop Wallpaper policy on the right side.
Select the Enabled option, and then type the path of the image you want to set as a default background. Click Apply and OK.
Right click on the Start Menu Button and select command prompt or command prompt (Admin) to open the CMD window. Type: gpupdate /force and hit ENTER.